What is a surge protection device and do I need one?
What an SPD in a consumer unit actually protects against, how it differs from a plug-in surge strip, and the households where it genuinely earns its cost.
- Difficulty
- beginner
- Time
- 12 min
- Read
- 3 min
Short answer
An SPD is a module in the consumer unit that clamps transient overvoltage from lightning or switching events before it reaches your circuits. It is now standard in new and replacement boards, and it makes most sense where there are overhead supply lines, a lot of expensive fixed electronics, or a heat pump or EV charger. Retrofitting one is consumer unit work and needs an electrician.
A surge is not the same thing as a power cut or a brownout. It is a spike lasting microseconds, typically from a nearby lightning strike coupling into the network or from large loads switching on the local grid, and it is far too fast for any fuse or RCD to notice. It is also the thing that quietly kills routers, boiler PCBs and induction hobs.
Step by step
- Understand the two places surge protection lives.A Type 2 SPD in the consumer unit protects the whole installation at the origin. A plug-in surge strip protects only what is plugged into it and only against what is left after the main event.
- Know what it does not do.An SPD does nothing about power cuts, voltage sag when the shower runs, or the slow deterioration that causes tripping. It is not a substitute for an RCD and it does not protect against direct lightning strike to the building.
- Assess your exposure honestly.Overhead supply lines rather than underground cable, rural locations, and areas with frequent thunderstorms carry more risk. So does having a lot of value on the fixed wiring: heat pump, EV chargepoint, solar inverter, boiler with an electronic control board, integrated appliances.
- Check whether you already have one.Boards installed from around 2019 usually contain one. It is a module in the consumer unit with a small indicator window, green when healthy and red when the device has absorbed its last surge and needs replacing.
- Check the indicator occasionally.An SPD is a consumable. Once the window shows red it has done its job and is no longer protecting anything until it is replaced. Look at it when you test the RCD every three months.
- Treat retrofit as consumer unit work.Fitting an SPD means working inside the consumer unit, which is not owner work. It is often cheap to add during any other board work, and disproportionately expensive as a standalone visit.
- Add plug-in protection for the things you actually care about.A decent surge-protected strip on a computer, a TV or a network cabinet is cheap and complements a board-level SPD. Check the joule rating and that it has a status light, and remember it is still an extension lead subject to the same load rules.
Tips
- Data cables carry surges too. A router protected on its mains lead but not on its incoming line can still be destroyed through the line.
- If the board already has an SPD, note where its indicator window is so you actually look at it.
- Surge strips wear out silently. A ten-year-old one with no status light is probably a plain extension lead now.
Common mistakes
- Assuming a surge strip protects the whole house — It protects only the sockets on that strip. Boilers, hobs, showers and lighting are on fixed wiring and see the full transient.
- Treating an SPD as an alternative to RCD protection — They address entirely different hazards. An SPD protects equipment from overvoltage; an RCD protects people from earth fault current.
If it doesn't work
SPD indicator has gone red
Cause: The device has absorbed surges up to its capacity and has disconnected itself — Fix: It needs replacing by an electrician. Until then the installation has no surge protection.
Electronics keep failing in the same house
Cause: Possible repeated transients, or possibly a wiring or earthing problem — Fix: Worth an EICR before assuming surges — poor neutral connections cause voltage swings that look like surge damage.
Questions people ask
Is an SPD compulsory?
The current wiring regulations require a risk assessment, and in practice most electricians fit one in any new or replacement consumer unit because the assessment usually favours it and the cost is small in context. It is not something existing installations must be retrofitted with.
Will it stop my lights flickering?
No. Flicker is a connection, load or bulb-compatibility problem, and an SPD has nothing to do with any of them.
Does it help with a power cut?
Not at all. For continuity of supply you need a UPS, which is a completely different product solving a completely different problem.